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Deposits of near pure silica on Mars were formed by volcanic vapours or hot-spring-type events crossing through soil and could contain traces of past life, scientists say.

The silica, detected in 2007 by the NASA rover Spirit is fully described in today's issue of the journal Science.

"On Earth, hydrothermal deposits teem with life and the associated silica deposits typically contain fossil remains of microbes," says study team member astrobiologist Professor Jack Farmer of the Arizona State University (ASU) in Tempe.

"What we can say is that this was once a habitable environment where liquid water and the energy needed for life were present," he says.

Making such pure silica requires a lot of water, says co-author Dr Steven Ruff also of ASU.

"On Earth, the only way to have this kind of silica enrichment is by hot water reacting with rocks," he says.

The combination of geothermal heat and water produces a hydrothermal system like that which powers the hot springs, geysers, mudpots, and fumaroles (steam vents) of Yellowstone National Park in the US.

These hydrothermal systems generally yield silica and other minerals as heated groundwater rises, cools, and gives off dissolved gases.

"If there were organisms living [on Mars]," says Farmer, "our terrestrial experience shows that microbes can easily be entrapped and preserved in the deposits."

Silica is an excellent medium for capturing and preserving traces of microbial life, says Farmer.

NASA landed Spirit and the other Mars rover, Opportunity, on opposite sides of Mars in January 2004 to scope out rocks showing the presence of water.

Now the rovers are more than four Earth years into a trek designed to go just three months.

While dust has collected on their solar panels and they have had some mechanical wear, both are still exploring.